Mass transfer media and method of operation

ABSTRACT

The invention is concerned with mass transfer media or devices such as are used to obtain effective contact between two fluids, for example in fractionating columns, washers, scrubbers, countercurrent liquid-liquid extractors and reactors. The invention is particularly concerned with a mass transfer device of the dished helical fin type which is rotated during operation with the axis inclined to the vertical, and by providing a line or band of perforations extending around the fin or fins of the device in the outer half thereof ensures improved wetting of the underside of the fin or fins and consequently improves the effectiveness of the device.

United States Patent [72] Inventor Ralph W. King 1,273,030 7/1918Campbell 19Grosvenor Place. London, England 2,216,722 10/1940 Denson[21] App]. No. 719,357

FOREIGN PATENTS 698,246 10/1953 Great Britain................

Primary ExaminerTim R. Miles [22] Filed Apr. 8, 1968 [45] Patented Jan.12, 1971 [32] Priority Apr 14, 1967 [33] Great Britain Art0rneyDelio andMontgomery 2 [3i No. 17,237/67 [54] MASS TRANSFER MEDIA AND METHOD OFgfigg gw Figs. ABSTRACT: The invention is concerned with mass transfermedia or devices such as are used to obtain effective contact betweentwo fluids, for example in fractionating columns, washers, scrubbers,countercurrent liquid-liquid extractors and reactors. The invention isparticularly concerned with a mass transfer device of the dished helicalfin type which is rotated during operation with the axis inclined to thevertical, and by providing a line or band of perforations extendingaround the fin or fins of the device in the outer half thereof ensuresimproved wetting of the underside of the fin or fins and consequentlyimproves the effectiveness of the device.

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ATTORNEK miss TRANSFER MEDIA ND METHOD or OPERATION This invention isconcerned with mass transfer media, i.e. elements or devices of the kindused to obtain effective contact between two fluids, usually between aliquid and a gas or vapor, for example in fractionating columns,washers, scrubbers and also in countercurrent liquid-liquid extractorsin which a substance initially in solution in one liquid is transferredinto solution in a second liquid which is not appreciably miscible withthe first liquid. The invention is more'particularly concerned with mustsuch mass transfer media or elements of the dished helical typecomprised of one or more dished helical fins bounded on the inside andoutside by concentric cylindrical walls so as to form one or morecontinuous passages which, when seen as a cross section through theaxis, have the appearance of being substantially parallelogrammatic ortrapezoidal. The expression dished is intended to mean that, when theaxis of the helical fin is vertical, a point on the outer edge of thefin is higher than a point on the inner edge of the fin. In operationthe dished helical element is mounted with its longitudinal axisinclined to the vertical and rotates about this axis. Such a dishedhelical element and fractionating apparatus incorporating it aredescribed in British Pat. Specification No. 698,246 and in pendingBritish Pat. application No. 3315 8/65.

The invention is also concerned with fractionation and other apparatusincorporating a mass transfer medium of the dished helical fin type.

In operation as a mass transfer medium, the dished helical element isrotated slowly about its axis which is inclined at an angle to thevertical and liquid is supplied continuously to the top of the elementwhilst gas, vapor. or a second immiscible liquid of lower density passesupwardly through the helical passage or passages formed by the fin orfins and the cylindrical retaining walls. The liquid supplied to the topof the helical elements tends to form pools on the top surface of theelement on the side to which its axis is tilted, but rotation of theelement causes the pools to move over the surface and down the element,so that the upper surfaces of the fin or fins of the eleof the element.

Though in the known constructions the entire upper surfacemost of theliquid passes through the pores or perforations rather close to theinner cylinder, so that the outer part of the fin is not effectivelywetted.

Careful observation of the flow of coloured liquids over the surface ofa rotating fractionating element the axis of which was tilted in theabove manner has. revealed a radial flow pattern. This is additional toand superimposed on the flow of liquid pools down the helix.

A continuous outward flow of liquid occurs from the inner boundary ofthe fin to the outer boundary near the line of maximum depth forstationary pools.

Whilst moving outward, the liquid is dragged by rotation of the fin to ahigher point where the radial slope of the fin is inward. The liquidthen flows back over'the fin to the inner boundary, and along the innerboundary to its starting point (disregarding movement in a helicaldirection with respect to the moving fin).

At low speeds of rotation the inward and outward flows merge and arediscernible mainly bythe movement of liquid at the edge of the pools. Asthe speed of rotation increases, the streams of liquid flowing outwardand inward separate; as the speed is further increased, the liquidflowing outward may stop short before reaching the outer boundary of thefin and ment are completely wetted by the liquid once per revolutionbegin to return towards the center. This is noticeable when the volumeof liquid is small or the surface is rough.

It has now been found that both upper and lower surfaces of the fin orfins of a dished helical element, rotating as described above, can bewetted to obtain increased efficiency in operation by providing a lineor band of perforations extending around each fin in the outer half ofthe fin, the area of the fin between the perforations and the inner edgeof the fin being substantially free of perforations.

The line or bank of perforations should occupy a strip of narrow widthand preferably be close to the outer edge of the fin and spacedtherefrom. The perforations may, however, be cut out of the outer edgeof the fin sothat they have open ends along said outer edge. Byappropriate choice of the size of the perforations it can be ensuredthat, liquid flowing outwardly along the upper surface of the fin willpass through the perforations and along the underside of the fin to theinner boundary of the fin. Within a certain range of rotational speedsthis return is regular and positive, and depending on the size 'of theperforations little, if any, drip will take place through theperforations to the upper fin surface lying below.

By locating the perforations at or close to the outer edge of the fin,the wetted area will constitute most of the fin area. Thus, the line orband of perforations should desirably be located in the outer quarter ofthe width of the fin and preferably in the outer one-tenth of the width.Thus with a fin 75 cms. in width, the.perforationspreferably lie in aband or strip of width 0.75 cms. extending around the fin inwardly fromthe outer edge thereof. The perforations are preferably so arrangedthat, during the rotation of the fin, liquid flowing over the outersurface of the fin towards the outer edge thereof will not bypass theperforations.

The perforations should be large enough to allow the liquid reachingthem to pass through, but not so large as to allow the about 6 mms. longand they are desirably set at an angle to the radius.

Hitherto fractionating or otherapparatus incorporating a dished helicalelement of the type referred to has been operated with an angle of tiltof the element equal to the angle of the fin, i.'e. the angle between astraight generating line on the surface of the fin and a plane at rightangles to the axis of It has now been found that by increasing the angleof tilt of the dished helical element by up to 15, and preferably byfrom 2 to 4, an improved performance is obtained, especially whenhelical fins having the perforations described above are used.

It is believed that the additional tilt compensates for the centripetalforce on the i liquid caused by the frictional drag on the liquid by thesurface of the rotating fin. This drag increases with the linearvelocity of the fin'on proceeding from the inner to the outer radius ofthe fin. Though the magnitude of the additional tilt required is notcritical, it has been found that at least 2 to 3 are necessary to securethe desired improvement.

The dished helical element or fin mentioned herein may also be referredto as a coned helical element or fin.

One embodiment of a dished helical mass transfer element according tothe invention incorporated in a fractionator is shown in theaccompanying drawings in which:

FIG. 1 is a longitudinal section through the fractionator;

FIG. 2 is is a fragmentary perspective view illustrating the dishedhelical fin element of the fractionator of FIG. 1, showing the bank ofperforations close to the outer edges of the fins; and

of a reflux Condenser 10, a reflux splitterll, a helical fin assembly l2and a reboil section 13 mounted one below the other in a housing 14formed of separate shell members 15, 16

and 17 bolted together and a closure plate 18 at the top thereof.

The condenser comprises cooling coils 10 19 provided with a liquid inlet20 and a liquid outlet (not shown) extending through the plate 18, thecoils 19 being disposed in an annular space between the shell 17 and acylindrical baffle 21 attached toa rotatable hollow shaft 22 extendingalong the axis of housing 14 by a boss 23, so that the baffle 21 willrotate with the shaft 22. q

Below the cooling coils 19 is a stationary annular collecting trough 24attached to the outer shell 17 and comprising a cone-shaped outer wall25 with an inner cylindrical wall 26 surrounding and spaced from theshaft 22. A nozzle 27 extends from and communicates with the collectingtrough 24, through which nozzle liquid collected by the trough 24 willflow.

The reflux splitter 11 is formed of two flat discs 28 and 29 in contactwith a thin spacing sheet 30 of Teflon or other material having a lowcoefficient of friction. The spacing sheet 30 may be replaced by acoating on one of the discs. Each of the discs 28 and 29 has fouropenings (not shown) therein, disposed symmetrically in the outer halfof the disc. Means (not shown) are provided for moving the discs 28 and29. relatively to one another and setting them in a position in whichthe openings in the one disc overlap those in the other disc to adesired extent. The reflux splitter 11 is mounted to rotate with shaft22. 1

Mounted below the refluxsplitter 11 in line with the nozzle 27 is acollecting dish or tundish.3ladaptedto receive all the liquid fromnozzle 27 which passes through the openings in the discs in the refluxsplitter 11 as they rotate beneath the nozzle 27. An outlet pipe 32 fromthe tn tundish 31 passes out through the shell member 17;

The openings in the discs28and 29 of the reflux splitter 11 are suchthat by relative movement of the discs the reflux ratio may be variedfrom 1:1 to infinity, i.e., from half to none of the condensate fromnozzle 27 may be permitted to pass to the tundish 31, the remainderflowing downwardly over the free edge of the reflux splitter. Thus thefractionator may be operated under total reflux downto a reflux ratio of1:1.

Also mounted on the shaft 22 below the reflux splitter 11 is the helicalfin s assembly 12 which constitutes the mass transfer or liquid vaporcontacting device, i.e. the fractionating element, of the fractionator.

The helical fin assembly 12 is composed of a two-start helix mountedbetween innerandouter cylinders 33 and 34, respectively 3 inches and 10inches in diameter, the inner cylinder 33 being attached to the shaft 22bymeans of bosses 35, 36, and the outer cylinder 34 being attached tothe inner cylinder by arms 37 so that the whole assembly rotates withthe shaft 22.

The outer cylinder 34 extends downwardly beyond the fin assembly to forma skirt or shroud 40 surrounding a cylinder 41, and to provide a liquidseal between the skirt; 40 and the cylinder 41 and also between theskirt40 and the outer shell member 16. A deflecting ring 42 projecting fromthe shell member 17 inwardly beyond the cylinder 34 prevents liquidfalling down between cylinder 34 and the shell member 16.

The shaft 22 extends don down to the bottom of the fractionator througha support bearing 43 carried by arms a 44 and carries a stirrer 45 atits lower end.

The reboil section l3is heated electrically by electric wiring (not soshown) surrounding the section and encased in heat insulating material(notshown).

Electric heating means encased in heat insulating material may also beprovided for that part of the fractionator between the reboiler and thecondenser to compensate for heat losses and to assist rapid startup.

The shaft 22 passes through a gland box 46 fitted in the plate 18 andthrough a bearing 47 and is fitted with a pulley 48 adapted to receive adriving belt for driving the shaft.

The housing 14 is provided with a gas outlet 49 through which vacuum maybe applied to the interior and/or noncondensables vented, a feed inlet50, a residue drain 51 and a liquid seal drain 52. The liquid or producttakeoff line isthe pipe 32.

In operation, the fractionator is inclined at an angle to the vertical.Vapor from the reboil section 13 passes up through the rotating finassembly 12 andthe reflux splitter 11 and condenses in condenser 10. Theliquid condensate runs down into the collecting trough 24 and throughthe nozzle 27 towards the rotating reflux splitter 11;? part passingthrough the openings in the discs 28, 29 of reflux splitter 11 and partcontacting the solid part of the discs. That part which passes throughthe openings is collected in the tundish 31 and passes out through pipe32. That part which falls on the solid part of the discs runs off theouter edge of the discs to drop down on to the upper turns of thehelical fin assembly 12 and then runs down around the finsto the reboilsection 13. In operation, a liquid seal of condensed vapor forms betweenthe skirt 40 and cylinder 41.

The fractionator described has a low pressure drop and mayadvantageously be used for distilling heat sensitive materials.

The dished helical fin element 12 is formed by attaching two continuouscurved helical fins 53'and 54 to the inner cylinder or cone 33. Theouter cylinder 34 fits tightly around the fins. Each fin comprises 28complete turns and has a pitch of one inch so that the distance betweenfins (less thickness of the metal) is one-half inch, measured in a lineparallel to the axis. The angle of fin or cone, i.e. the angle between astraight generating line on the surface of the fins and a plane are atright angles to the axis is 20. The fins may be made by welding togethersplit annular discs of 24 gauge sheet metal. The none nonperforated areaof the fins is embossed to provide a dimpled effect.

The I helical fins a helical fins 53 and 54 are provided with three rowsof holes 55 of diameter 0.144 inches in circles of pitch diameters 9.4,8.4 and 7.4 inches as shown in FlG. 2. Each complete turn of each helixcontains 43 holes per row or 129 holes in total. An alternative form orperforation is shown in FIG. 3 in which the perforations are in the formof slots 56 arranged at an angle of 45 to the radius.

The benefit of providing a narrow band of perforations close to theouter edge of the fins of a dished helical fin fractionating element ofa fractionator is illustrated by the following example. The improvedresult obtained by increasing the angle of tilt is also illustrated.

EXAMPLE The apparatus used was the fractionator shown in FIG. 1 anddescribed above. That part of the fractionator between the reboilsection 13 and the condenser 10 was lagged and compensated by electricheating on the outside to prevent heat losses. The space the condenser10 was connected via outlet 49 and an automatic control valve, whichallowed the absolute pressure to be automatically controlled, to avacuum pump. Suitable instruments were fitted to measure the temperatureand pressure of vapor above and below the element 12. The seal ring atthe lower end of shroud 40 was filled with liquid during operation. Thefractionator was mounted in a frame enabling it to be tilted with itsaxis at various angles to the vertical.

Three sets of experiments were carried out, in the first of which thefins of the helical fin element 12 were devoid of perforations but wereotherwise identical with those described above in connection withtheaccompanying drawings. In the second and third sets of experimentsthe fins of the helical fin element were provided with the three rows ofholes of the size and disposition specified above in connection withFIG. 2.

In each experiment a mixture of 4 litres of pure diethyl phthalate and 1litre of pure dimethyl phthalate was fed to the kettle or reboiler 13through inlet 50 and heat was applied to the reboiler so vapor passed upthrough the helical passages of element 12 to the condenser 10 where itwas totally condensed, the liquid returning through the passages of theelement 12 to the rebont All experiments were carried out at a condenserpressure between 5.5 and 6.5 torr at a constant vapor top temperature of138 C.

The latent heat of vaporization of the mixture under these conditionswas calculated as 130 B.t.u. per pound and the relative volatility ofthe mixture as 1.45, and these figures were used in all subsequentcalculations.

The efficiency of the fractionator was determined under each set ofconditions by withdrawing and analyzing samples of the overheadcondensate and kettle liquid under d conditions approximating to totalreflux." The efficiencies were calculated as number of theoreticalplates at total reflux" by the well-known Fenske Underwood equation.

The vapor throughput in all experiments was maintained at 28 lbs/hour lpercent) by adjustment of the heat input to the kettle. This was checkedperiodically by measuring the rate of flow and temperature rise of thewater entering and leaving the condenser.

The first set of experiments (with the nonperforated fins) comprised aseries of experiments carried out at rotational speeds from to 60r.p.m., with the axis inclined at angles of 20, 23 and 26 to thevertical. The direction of rotation was creasing the angle of tilt from20 to 23 raised the maximum efficiency from 4.9 to 5.5 theoreticalplates (approximately 12 percent). Further increase in the angle of tiltfrom 23 to 26 produced no further increase in fractionating efficiency,although the pressure drop was somewhat increased. The pressure dropunder conditions of maximum efficiency at an angle of 23 was 0.51 torrper theoretical plate.

The second set of experiments in which the perforated fins were usedcomprised a similar series of experiments. The fractionating efficiencywas measured at a series of rotational speeds as before. The effect ofthe perforations on the fractionating efficiency at an angle of tilt of23 was to raise the maximum efficiency to 8 theoretical plates, thisnumber appearing as a sharper maximum at a rotational speed of r.p.m.Changing the angle of tilt to 20 and 26 with the perforated fins had thesame relative effect on the fractionating efficiency as before. Thepressure drop per theoretical plate at a speed of l0 r.p.m. and an angleof 23 amounted to 0.41 torr.

The increase in fractionating efficiency resulting from the rows ofperforations near the periphery was undoubtedly due to the wetting ofthe underside of the fins which now occurred. This wetting effect wasshown in a series of subsidiary experiments in which the fractionatingelement was removed from the casing and its close fitting cylindricalwrapper, and rotated in an inclined position whilst a stream of watercontaining a small amount of detergent was directed on to the top of theelement. V

In the third set of experiments, also using the perforated fins, thedirection of rotation was reversed so as to cause the pools of liquid onthe fins to move upwards instead of downwards. The effect of this was toincrease very considerably the amount of liquid passing through. theholes and flowing across the underside of the fins. The element-nowshowed a maximum efficiency of 7.6 theoretical plates at an angle oftilt of 23 and at the same speed of rotation as before,

' tions, although the degree of wetting of all surfaces of the heli-'cal passages was best with the reversed direction of rotation.

Reverse rotation would not normally be used for fractionators. but maybe advantageous in cases where it is very important that all surfaces ofthe helical element be fully and positively wetted and flushed. e.g. incases where both mass transfer and chemical reaction takes place. Atypical case of this kind is the sulfonation of an organic liquid inwhich the liquid flows downwardly through the helical element in contactwith an upwardly flowing airstream containing sulfur trioxide.

in operating apparatus incorporating the dished helical fin element, thespeed of rotation of the dished helical fin element should be below thatat which a substantial part of the liquid is removed from the fins tothe inner surface of the oute cylinder by centrifugal force.

Though in the apparatus shown in HO. 1 an outer cylinder 34 is fitted tothe helical fin element, this cylinder may be omitted an and the finsmay instead extend close the the wall of the housing or outer casing 16without actually making contact with the wall, the outer casing 16 thenconstituting the outer cylinder of the helical fin assembly.

The invention has been described above in detail as applied tofractionating apparatus, but it has been found to be of advantagegenerally for any apparatus in which effective contact between twofluids is required. Thus, the dished helical fin ele-' ment of theinvention may advantageously be used in scrub: bers and washers toobtain effective contact between a liquid and a vapor, in countercurrentliquid-liquid extractors in which a substance in solution in one liquidis transferred into solution in a second liquid which is not appreciablymiscible with the first liquid, in reactors in which mass transfer of asubstance between two fluid phases is followed by chemical reaction withthe transferred substance, and in other apparatus for performing similaroperations. I

Examples of apparatus in which both mass transfer and chemical reactiontake place are oxygenators for the oxygenation of blood and sulfonationreactors for carrying out the sulfonation reaction mentioned above.

Furthermore, improved results are obtained not only when compared withnonperforated fins as shown by the above experiments but also whencompared with fins uniformly perforated over the whole of their areas.

I claim:

1. For apparatus of the kind referred to in which mass transfer betweentwo fluids is obtained by bringing the fluids into effective contactwith one another, a mass transfer device for effecting such contactconsisting of a dished helical fin element comprising at least onehelical fin, characterized in that each helical fin has at least one rowof perforations extending around the fin in the outer half thereof, thearea of the fin between the perforations and the inner edge thereofbeing substantially free of perforations.

2. A mass transfer device according to claim 1, in which theperforations are located in the outer quarter of the width of the fin.

3. A mass transfer device according to claim 2, in which theperforations are located in the outer one-tenth of the width of the fin.

4. A mass transfer device according to claim 1, in which the fin isprovided with one or more rows of circular perforations.

5. A mass transfer device according to claim 1, in which the fin isprovided with a row of perforations in the form of slots.

6. A mass transfer device according to claim 5, in which the slots areinclined at an angle to the radius of the fin.

7. Fractionating or other apparatus. including the mass transfer deviceof claim I mounted for rotation about its axis within the apparatus.

8. A method of operating apparatus according to claim 7 with the masstransfer device rotating continuously, in which the apparatus is tiltedso that the mass transfer device rotates with its axis at an angle tothe vertical equal to the angle of the fin or greater than the angle ofthe fin by up to 15.

9. A method according to claim 8, in which the angle offtilt is greaterthan the angle of the fin by from 2 to 4.

10. A method according to claim 8,'in which the direction of rotation issuch as to cause liquid on the upper surface of the fin to movedownwards. i

11. A method according to claim 8, in which the direction of rotation issuch asto cause liquid on the upper surface of the fin to move upwards.

1. For apparatus of the kind referred to in which mass transfer betweentwo fluids is obtained by bringing the fluids into effective contactwith one another, a mass transfer device for effecting such contactconsisting of a dished helical fin element comprising at least onehelical fin, characterized in that each helical fin has at least one rowof perforations extending around the fin in the outer half thereof, thearea of the fin between the perforations and the inner edge thereofbeing substantially free of perforations.
 2. A mass transfer deviceaccording to claim 1, in which the perforations are located in the outerquarter of the width of the fin.
 3. A mass transfer device according toclaim 2, in which the perforations are located in the outer one-tenth ofthe width of the fin.
 4. A mass transfer device according to claim 1, inwhich the fin is provided with one or more rows of circularperforations.
 5. A mass transfer device according to claim 1, in whichthe fin is provided with a row of perforations in the form of slots. 6.A mass transfer device according to claim 5, in which the slots areinclined at an angle to the radius of the fin.
 7. Fractionating or otherapparatus including the mass transfer device of claim 1 mounted forrotation about its axis within the apparatus.
 8. A method of operatingapparatus according to claim 7 with the mass transfer device rotatingcontinuously, in which the apparatus is tilted so that the mass transferdevice rotates with its axis at an angle to the vertical equal to theangle of the fin or greater than the angle of the fin by up to 15*.
 9. Amethod according to claim 8, in which the angle of tilt is greater thanthe angle of the fin by from 2* to 4*.
 10. A method according to claim8, in which the direction of rotation is such as to cause liquid on theupper surface of the fin to move downwards.
 11. A method according toclaim 8, in which the direction of rotation is such as to cause liquidon the upper surface of the fin to move upwards.